With the recent widespread of such a photographic apparatus as a camera, photo-taking opportunities have also increased with the use of a silver halide photographic light sensitive material. According thereto, there have been increasing demands for further improvements in sensitivity and image quality. One of the dominant factors to the high sensitivity and high image-quality of a silver halide photographic light sensitive material is concerned with silver halide grains. Research and development of silver halide grains have also progressed so far in the field of the art, with the purpose of making the sensitivity and image quality thereof to be higher. However, there have been a limit to satisfy both of high sensitivity and high image-quality, because the sensitivity thereof tends to be lowered when the size of silver halide grains is made smaller so as to improve the image quality, as has usally been attempted in the art.
To achieve further improvement in sensitivity and image quality, techniques for enhancement of a ratio of sensitivity/grain size per a grain have been studied so far; for example, the use of tabular grains was disclosed in JP-A 58-111935/1983, 58-111936/1983, 58-111938/1983, 58-113927/1983 and 59-99433/1984 (the term "JP-A" means an "unexpected published Japanese patent application"). Comparing to octaheral, tetradecahedral or hexahedral crystal grains, socalled, regular crystal silver halide grains, these tabular grains have larger surface area per volume so that more sensitizing dyes are adsorbed to the silver halide surface to increase advantageously the sensitivity.
There were described a technique for provide a core having high silver iodide content within silver halide tabular grains in JP-A 63-92942/1988 and the use of silver halide tabular grains having a ratio of grain thickness to the longest distance between twin planes of 5 or more in JP-A 63-163451/1988, each showing the effects thereof on the sensitivity and graininess.
There were disclosed the use of silver halide tabular grains having substantially a layered structure in the direction parallel to two opposed major faces and silver halide tabular grains having substantially a layer structure divided by a plane parallel to two opposed major faces and comprising the outermost layer having a silver iodide content, by not less than 1 mol %, more than the average overall silver iodide content in JP-A 63-92942/1988 and JP-A 1-279237/1989, respectively. In addition thereto, JP-A 1-183644/1989 discloses silver iodohalide tabular grains having a uniform iodide distribution.
Furthermore, as a technique for incresing sensitivity, there are disclosed the use of silver halide tabular grains having dislocation lines localized in a specified position within the grain or concentrated in the vicinity of the corner in JP-A 63-220238/1988 and 3-175440/1991, respectively, and those having a definite core/shell structure or three-layered core/shell structure in JP 3-18695/1991 and 3-31245/1991 (the term "JP" means examined published Japanese patent).
It is, however, difficult to accomplish both high-sensitivity and high image-quality by these prior arts and insufficient to satisfy the demands in modern photographic light sensitive material; therefore development of superior technique has been desired.
In one aspect, there has been known a technique of metal-doping, whereby carrier-control is attempted. The metal-doping is a technique of occluding a polyvalent metal compound within silver halide grains to improve photographic characteristics; there are cited doping of an iridium compound as disclosed in JP-A 62-7042/1987 and an iron compound doping as disclosed in JP-A 1-121844/1989.
Further, there has been known reduction sensitization as a technique for attaining high-sensitization of silver halide. With respect to the reduction sensitization, an optimally reduction-sensitized nucleus has been considered to attribute the sensitization when exposed to light, through the following reaction, as disclosed in Journal Photographic Science Vol. 25, pages 19-27 (1977); Photographic Science and Engineering, Vol. 23, pages 113-117 (1979); Photographishe Korrespondenz Vol. 1, page 20 (1957) and Photographic Science and Engineering, Vol. 19, pages 49-55 (1975). EQU AgX+h.nu..fwdarw.e.sup.- +h.sup.+ ( 1) EQU Ag.sub.2 +h.sup.+ .fwdarw.Ag.sup.+ +Ag (2) EQU Ag.fwdarw.Ag.sup.+ +e.sup.- ( 3)
where h.sup.+ and e.sup.- represent a free positive hole and free electron produced on exposure, h.nu. represents a photon and Ag.sub.2 represents a reduction sensitization nucleus.
According to Photographic Science and Engineering, Vol. 16 pages 35-42 (1971) and ibid Vol. 23 pages 113-117 (1979), it is contemplated that the reduction sensitization nucleus is inherently capable of trapping not only positive hole but also an electron and at the present time, therefore, the mechanism thereof cannot be sufficiently explained only based on the above theory.
The role of the reduction sensitization in the spectral sensitizing range of spectarally-sensitized silver halide grains in the form of being used in a silver halide photographic light sensitive material, which is different from that in a sensitivity range inherent to silver halide has been difficult to be expected because of complexity of a latent image-forming process.
In a spectrally sensitized silver halide emulsion, generally speaking, a dye absorbs light so that the initial step in the latent image-forming process, which is different from that in a inherent sensitivity range has been contemplated to be represented by the following (4), in place of (1) above-described. EQU Dye+h.nu..fwdarw.Dye.sup.+ +e.sup.- ( 4)
Probability as to whether a dye positive hole (Dye.sup.+) and electron (e.sup.-) are transferred to the silver halide grains depends largely on property of the dye. Regarding the dye positive hole, it has been considered, in general that a sensitization efficiency is better in the case when the dye positive hole is not transferred to the inside of the silver halide grains, as discussed in relation with an oxidation potential of the dye in Photographic Science and Engineering Vol. 24 pages 138-143 (1980).
It was suggested in Collective Abstracts of International Congress of Photographic Science pages 159-162 (1978) and Photographic Science and Engineering Vol. 17 pages 235-244 (1973) that a sensitizing dye of which positive hole is liable to be trapped on the surface of the silver halide grains is to bleach a fogging nucleus or reduction sensitization nucleus. In a conventional surface latent image-forming emulsion, therefore, the surface latent image is presumed to be bleached, leading to desensitization.
As mentioned so far, it has not been known as yet which the reduction sensitization is applied the surface of silver halide grains or the inside thereof or what kind of a dye is to be combined therewith to display the effect.
As a method of applying the reduction sensitization practically to a silver halide emulsion, there have been known some examples such as application of the reduction sensitization to the surface of the grains or to the grain in a process of preparation thereof, and grain growth from redution-sensitized seed grains.
A method in which reduction sensitization is applied to the grain surface, when combined with other sensitization (gold or sulfur sensitization), results in a remarkable increase in fog and is not suited for practical use. On the other hand, a method in which the reduction sensitization applied to the grains in a process of the preparation thereof was reported not to display such disadvantage as above-mentioned. This method, as described, for example, in JP-A 48-87825/1973 or 57-179835/1982, however, is concerned with an improvement in inherent sensitivity, not in spectral sensitivity.
There was described in JP-A 58-127920/1983 an improvement in spectral sensitivity when silver halide grains are internally reduction-sensitized, however, the effect thereof was limited to a spectral sensitizing dye having an oxidation potential of 0.5 V or more.
Today, in the market, there is an increasing tendency to regard stability in quality so important that latent image stability has been more in demand than ever. As a result of surveys of consumers' application of photographic film, the fact revealed that photographic films have been used under severe conditions after exposure, for example, a film is kept in the camera without being developed, or the camera or film was left near a window or in a car during the hot summer season. It is recognized to be important in actual practice that after being exposed, a latent image must be stably preserved even under such severe conditions. However, in the prior arts, there have not been viable means to meet such requirements.